Its suddenness deprived me of the power to exert myself for his succour.

I wildly gazed around me, in hopes of seeing him rise.

After some time my attention was drawn, by the sound of agitation in the water, to a considerable distance.
It was too dark for any thing to be distinctly seen.

There was no cry for help.

The noise was like that of one vigorously struggling for a moment, and then sinking to the bottom.